How do digital-goods merchants fight fraud and chargebacks?

This is a toughie. It all depends on what sort of digital goods are being sold. If it is something like a SaaS service, where access to the product and/or service can be denied, then there is not much fraud in that arena.

However, if a digital good can be downloaded and be used on its own, without some back-end activation or being tethered to some system, then the chances of fraud are relatively higher.

Most of the time fraudsters will buy, and then after 2-3 days request a reversal of the transaction with their credit card company, citing that they never placed the order. As a merchant, you would usually have to provide proof of delivery, but even then the fraudster can claim, they never made the charge. Your word against theirs. They usually win.

There are many services that will reduce fraud for you, by matching things like AVS to the IP, and 100s of other fraud parameters, etc. They all result in lower frauds/chargebacks, but do not 100% eliminate it. Its cost of doing business I guess.

I would really like the wide scale adoption of irreversible payment systems like Bitcoin and Ripple (payment network) (especially the latter).